Fluid catalytic cracking plays an essential role in the conversion of high boiling point hydrocarbons to more valuable lighter hydrocarbons such as gasoline. There is a trend towards processing more economical, heavier feedstocks such as atmospheric resid rather than more traditional feedstocks such as vacuum gas oil. Before being fed to an FCC unit, however, these heavier feedstocks must be treated to remove contaminants detrimental to successful operation of an FCC process. Sulfur, nitrogen and metals (e.g., vanadium and nickel) all pose potential problems for FCC operation. Basic nitrogen compounds are particularly troublesome for FCC catalysts due to poisoning of acidic zeolite sites.
Hydroprocessing treatment to remove sulfur, nitrogen and metals typically consists of exposing the feedstock to a series of catalyst beds at elevated temperatures and pressures in the presence of hydrogen. The purpose of the initial catalyst bed is primarily the removal of metal containing compounds from the feedstock. This serves as protection for later catalyst beds, whose primary purpose is the removal of sulfur and nitrogen. These beds have limited tolerance for metals before deactivation occurs. This invention relates specifically to these later catalyst beds whose main purpose is sulfur and nitrogen removal. It should be noted that some metals will make it to these beds and that metals tolerance is still an important factor that can shorten catalyst life.
In practice, hydroprocessing operations target a specific sulfur level in the product. Reactor temperature is increased or decreased to hit this sulfur target. Even if a catalyst gives improved sulfur and nitrogen removal, in practice the reactor temperature would be decreased to hit the target sulfur level. If increased nitrogen removal is desired, it is advantageous to use a catalyst that is selective for nitrogen removal (i.e.—higher nitrogen removal versus sulfur removal ratio).
In the hydroprocessing process, hydrocarbon feedstocks are contacted with a hydroconversion catalyst in the presence of hydrogen at elevated pressure and temperature. Catalysts used in hydroprocessing processes generally comprise catalytically active metals from Groups VIB (Group 6) and Group VIII (Groups 8, 9 and 10) of The Periodic Table and are typically supported on a support, typically made predominately of alumina. The operating conditions are typically driven to maximize HDS, and typical operating conditions have included a reaction zone temperature of 300° C. to 500° C. a hydrogen partial pressure of 3 to 25 MPa, a hydrogen feed rate of 400 to 3000 normal liters of hydrogen gas per liter (N L/L) of oil feed, and a catalyst such as nickel or cobalt and molybdenum or tungsten on a predominately alumina support. However, since optimum HDN temperatures are not the same as optimum HDS temperatures, it is an advantage if the selectivity for HDN can be raised for a given HDS level of activity.
To this end, there remains a need to develop catalyst compositions which provide good hydrodesulfurization of heavy oil and residuum feedstocks while simultaneously providing improved HDN during a hydroprocessing process.